Optical modulator and optical modulation control method

ABSTRACT

An optical modulator includes: a modulator including an optical waveguide provided in a semiconductor substrate having an electro-optical effect and an electrode to apply an electric field depending on a bias voltage and a modulation signal to the optical waveguide; a driver circuit to generate a modulation signal in accordance with an input signal; a superimposer to superimpose a reference signal on the bias voltage, the reference signal having lower frequency than the modulation signal; and a controller to control a bias voltage in a direction orthogonal to a modulation direction of the modulator based on the frequency component of the reference signal extracted from a modulated optical signal generated by the modulator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2010-281086, filed on Dec. 16,2010 and the prior Japanese Patent Application No. 2011-061692, filed onMar. 18, 2011, the entire contents of which are incorporated herein byreference.

FIELD

The embodiments described in this application are related to an opticalmodulator and an optical modulation control method, and may be appliedto the optical modulation using, for example, a semiconductorMach-Zehnder modulator (SMZM).

BACKGROUND

Recently, a semiconductor Mach-Zehnder modulator (SMZM) has beencommercialized as an optical modulator. The SMZM is more easily realizedas a small device than an LN modulator etc., and has a broaderwavelength band characteristic than an electro-absorption modulator (EAmodulator).

As a related technique, an optical semiconductor device in which asemiconductor laser and a Mach-Zehnder modulator are integrated in thedirection of optical waves guided on the same semiconductor substratehas been proposed (for example, Japanese Laid-open Patent PublicationNo. 2009-198881).

As another related technique, the following optical modulator has beenproposed. That is, the optical modulator includes: optical interferencemeans for branching input light into two optical waveguides, combininglight beams which propagate the respective optical waveguides, andoutputting the combined light; phase modulating voltage supplying meansfor supplying a voltage for modulating of the phase of propagated lightto at least one optical waveguide in the two optical waveguides; directcurrent voltage supplying means for supplying a direct current voltageto at least one optical waveguide in the two optical waveguides; anddirect current control means for controlling the value of the directcurrent voltage supplied by the direct current voltage supplying meansdepending on the wavelength of the input light (for example, JapaneseLaid-open Patent Publication No. 2005-326548).

As a further related technique, the following optical transmitter hasbeen proposed. That is, the optical transmitter includes: a lightsource, a drive circuit for generating a drive voltage depending on aninput signal; an optical modulator for modulating the emitted light fromthe light source depending on the drive voltage, and converting theinput signal into an optical signal; and an operation pointstabilization circuit for detecting the drift of the operationcharacteristic curve of the optical modulator, and controlling theoptical modulator so that the operation point is placed in a specifiedposition with respect to the operation characteristic curve. The opticaltransmitter further includes an operation point shift circuit forshifting the operation point by half cycle on the operationcharacteristic curve according to an operation point switch signal (forexample, Japanese Laid-open Patent Publication No. 04-140712).

The SMZM includes a pair of optical waveguides. The input light from thelight source is branched and directed to the pair of optical waveguides.In addition, the SMZM also includes an electrode for supplying anelectric field to each optical waveguide. A drive signal generated froma data signal and a bias voltage are applied to each electrode. Then theSMZM generates a modulated optical signal by modulating the input lightwith the drive signal. In this case; a high quality modulated opticalsignal is generated by appropriately adjusting the drive amplitude (thatis, the amplitude of the drive signal) and the bias voltage.

However, the static characteristic of the SMZM indicates variance foreach device, and depends on the wavelength of input light. Therefore, todetermine in advance the optimum combination of a drive amplitude and abias voltage for each SMZM while considering the wavelength of inputlight, an enormously long time is taken. In addition, although theoptimum combination of a drive amplitude and a bias voltage isdetermined in advance for the SMZM, the static characteristic of theSMZM may be changed depending on the ambient temperature, aging, etc. Ifthe static characteristic of the SMZM changes, the quality of amodulated optical signal is degraded. For example, there occur thefold-back of an optical waveform, the degradation of an extinctionratio, the fluctuation of a cross point, the reduction of the apertureof an optical waveform, etc.

The static characteristic of the SMZM is different from that of a commonLN modulator. Therefore, although a method of adjusting the operatingstate of an LN modulator is introduced to the SMZM, it is, hard tooptimize the operating state of the SMZM.

SUMMARY

According to an aspect of the invention, an optical modulator includes:a modulator including an optical waveguide provided in a semiconductorsubstrate having an electro-optical effect and an electrode to apply anelectric field depending on a bias voltage and a modulation signal tothe optical waveguide; a driver circuit to generate a modulation signalin accordance with an input signal; a superimposer to superimpose areference signal on the bias voltage, the reference signal having lowerfrequency than the modulation signal; and a controller to control a biasvoltage in a direction orthogonal to a modulation direction of themodulator based on the frequency component of the reference signalextracted from a modulated optical signal generated by the modulator.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of an optical modulator according to thefirst embodiment;

FIG. 2 illustrates an example of an optical modulation control method;

FIG. 3 illustrates an example of a static characteristic and a drivingmethod of an SMZM;

FIG. 4 illustrates an example of a modulating operation in the Y-axisdirection;

FIG. 5 illustrates an example of a static characteristic of an SMZM;

FIG. 6 illustrates an optical output with respect to voltage in theY-axis direction;

FIG. 7 illustrates an optical output with respect to voltage in theX-axis direction;

FIG. 8 illustrates an example of an optical transmission moduleaccording to the second embodiment;

FIG. 9 illustrates an example of an operation when a low frequency issuperimposed on Y-axis direction bias;

FIG. 10 illustrates an example of V1bias, V2bias, and voltage waveformsin the X and Y-axis directions;

FIG. 11 illustrates an example of a control operation of a Y-axisdirection bias;

FIG. 12 illustrates an example of a bias voltage and a modulation signalin the Y-axis direction, and an optical output;

FIG. 13 illustrates an example of an operation when a low frequency issuperimposed on a drive amplitude control signal;

FIG. 14 illustrates an example of an amplitude control voltage, anin-phase modulation signal, a reversed-phase modulation signal, and avoltage in the Y-axis direction;

FIG. 15 illustrates an example of a control operation of the driveamplitude in the Y-axis direction;

FIG. 16 illustrates an example of an amplitude control voltage, a datasignal modulation signal, and an optical output;

FIG. 17 illustrates an example of an operation when a bias voltage inthe X-axis direction is modulated;

FIG. 18 illustrates an example of V1bias, V2bias, and voltage waveformsin the X and Y-axis directions;

FIG. 19 illustrates an example of a control operation of a X-axisdirection bias;

FIG. 20 illustrates an example a bias voltage in the X-axis directionand optical output power;

FIG. 21 illustrates an example of an optical transmission moduleaccording to the third embodiment;

FIG. 22 is a flowchart of an example of a control operation;

FIG. 23 illustrates an example of an optical transmission moduleaccording to the fourth embodiment;

FIG. 24 illustrates an example of an X-axis direction bias controller, aY-axis direction bias controller, and a bias controller according to thefifth embodiment;

FIG. 25 illustrates a configuration and operation of compared example;

FIG. 26 illustrates a configuration of the optical transmission moduleprovided with a QPSK modulator;

FIG. 27 illustrates a configuration of the QPSK modulator illustrated inFIG. 26;

FIGS. 28A and 28B illustrate a control system of the opticaltransmission module in FIG. 26; and

FIG. 29 is a flowchart of the method of controlling the opticaltransmission module in FIG. 26.

DESCRIPTION OF EMBODIMENTS

<First Embodiment>

In the first embodiment, the bias control not only in the Y-axisdirection but also in the X-axis direction is realized by feedbackcontrol (that is, automatic control). That is, the bias control isrealized without acquiring in advance data etc. indicating the optimumbias voltage. The X axis and the Y axis are described later.

FIG. 1 illustrates an example of an optical modulator according to thefirst embodiment. An optical modulator 2 illustrated in FIG. 1 is anexample of the optical modulator according to the present invention, andincludes a semiconductor Mach-Zehnder modulator (hereafter referred toas an SMZM) 4 as an optical modulator. The SMZM 4 is realized by using asemiconductor substrate having an electro-optical effect. The SMZM 4 isan example of an optical modulator, and modulates the phase oftransmission light according to the bias voltage and the modulationsignal applied to a signal electrode.

The optical modulator according to the embodiments of the presentinvention is not limited to a semiconductor modulator, but includes amodulator which provides optical absorption modulation when an opticalphase is modulated. FIG. 1 illustrates an example of a binary phasemodulation, but the present invention is not limited to this example.That is, the optical modulator according to the embodiments of thepresent invention is also applied to a multilevel phase modulator (forexample, a QPSK modulator). A multilevel phase modulator is realized byproviding a plurality of optical modulators illustrated in FIG. 1.

The SMZM 4 includes a first optical waveguide 6 and a second opticalwaveguide 8. The optical waveguides 6 and 8 are formed in theabove-mentioned semiconductor substrate. The optical waveguides 6 and 8are formed, for example, parallel to each other. Input light is guidedto the SMZM 4. The input light is carrier wave for transmission of asignal, and may be CW (continuous wave) light generated by, for example,a direct current light source. The light source is, for example, a laserlight source. However, an optical signal output from an opticalmodulator may be input to the SMZM 4. The input light is branched by anoptical splitter 10, and guided to the optical waveguides 6 and 8. Thelightwaves which has passed the optical waveguides 6 and 8 are combinedby an optical combiner 12. Thus, a modulated optical signalcorresponding to the modulation signal such as a data signal etc. isgenerated.

A first signal electrode 16 is provided for the first optical waveguide6. Also, a second signal electrode 18 is provided for the second opticalwaveguide 8. The optical waveguides 6 and 8 and the signal electrodes 16and 18 are formed to realize a Mach-Zehnder interferometer in thesemiconductor substrate having an electro-optical effect. The signalelectrodes 16 and 18 respectively apply the optical waveguides 6 and 8with electric fields depending on the bias voltage and the modulationsignal. As a result, the phases of the light which passes through theoptical waveguides 6 and 8 are modulated depending on the bias voltageand the modulation signal.

The signal electrode 16 is electrically coupled to a first inputterminal 20, and the signal electrode 18 is electrically coupled to asecond input terminal 22. An input voltage V1 is provided for the inputterminal 20, and the second input voltage V2 is provided for the inputterminal 22. Terminators 24 and 26 are electrically coupled to thesignal electrodes 16 and 18, respectively.

The optical modulator 2 is provided with a driver 28 and a controller 30as peripheral circuits for the SMZM 4. The driver 28 generates amodulation signal for driving the SMZM 4 from the input data signal. Themodulation signal includes an in-phase modulation signal V1 pp and areversed-phase modulation signal V2 pp. The reversed-phase modulationsignal V2 pp has a reversed phase with respect to the in-phasemodulation signal V1 pp. The controller 30 generates a first biasvoltage V1bias and a second bias voltage V2bias. An input voltage V1 isobtained by adding the bias voltage V1bias to the in-phase modulationsignal V1 pp, and an input voltage V2 is obtained by adding the biasvoltage V2bias to the reversed-phase modulation signal V2 pp. The inputvoltage V1 and V2 are generated using bias T circuits 32 and 33, andapplied to the signal electrodes 16 and 18 through the input terminals20 and 22, respectively. The bias T circuits 32 and 33 are electriccircuits including, for example, an inductor and a capacitor. Using thebias T circuits (32 and 33), the high frequency signals (V1 pp, V2 pp)are not affected by the respective direct current components (V1bias,V2bias), and the direct current components are not affected by therespective high frequency signals. The configuration and the operationof the bias T circuit are described in, for example, Japanese Laid-openPatent Publication No. 2007-109839.

When the input voltages V1 and V2 are applied to the signal electrodes16 and 18, the refractive indices of the optical waveguides 6 and 8 arechanged depending on the applied voltages by an electro-optical effect.The changes of the refractive indices modulate the phases of thetransmission light through the optical waveguides 6 and 8. That is, therefractive index of the first optical waveguide 6 changes depending onthe input voltage V1, and the refractive index of the optical waveguide8 changes depending on the input voltage V2. As a result, in eachoptical waveguide, an optical phase is modulated. For example, in theoptical waveguide 6, 0/−π modulation is performed on the input light,and in the optical waveguide 8, 0/π modulation is performed on the inputlight. In the phase modulation, push-pull drive (differential drive) maybe performed to suppress optical frequency chirp. As a result, a lowchirp modulated optical signal is generated, and output from the SMZM 4.

The driver 28 generates a modulation signal (in-phase modulation signalV1 pp and reversed-phase modulation signal V2 pp) for driving the SMZM4. The driver 28 adjusts the amplitude of the modulation signalaccording to an amplitude control signal Vc provided from the controller30. When the low frequency signal Lf is superimposed on the amplitudecontrol signal Vc, the amplitude of the modulation signal fluctuatesdepending on the frequency of the low frequency signal Lf. In thedescription below, the state of the amplitude of the modulation signalfluctuating depending on the frequency of the low frequency signal Lfmay be referred to as an “amplitude-modulation (by the low frequencysignal Lf)”. The amplitude of the modulation signal is referred to as a“drive amplitude (or modulation amplitude)”. The low frequency signal Lfis an example of a reference signal, and for example, a low frequencysmall signal (dithering signal) having a small amplitude of about 1 kHz.

The controller 30 controls the SMZM 4 and the driver 28. The controller30 controls the bias voltages V1bias and V2bias and the amplitudecontrol signal Vc according to the modulated optical signal output fromthe SMZM 4. In this case, the optical monitor 34 monitors the opticalsignal output from the SMZM 4, and the controller 30 performs thecontrol above according to the output of the optical monitor 34.

A low frequency modulator (low frequency signal generator) 36 generatesa low frequency signal Lf. The low frequency modulator 36 operates as(part of) a superimposer to superimpose a reference signal on the biasvoltage and the amplitude control signal Vc.

A phase detector 38 detects the frequency component (hereafter referredto as a low frequency component) of the low frequency signal Lf includedin the modulated optical signal output from the SMZM 4. That is, thephase detector 38 operates as a low frequency detector. In this case,the phase detector 38 detects the amplitude and the phase of the lowfrequency component in the output optical signal using the low frequencysignal Lf generated by the low frequency modulator 36. The amplitude andthe phase of the low frequency component in the output optical signaldepends on the operation condition (bias voltage and drive amplitude) ofthe SMZM 4. For example, when the operation state of the SMZM 4 isoptimized, the power or the amplitude of the low frequency componentincluded in the output optical signal is zero (FIGS. 12, 16, 20, etc.).Therefore, the controller 30 controls the bias voltage and/or driveamplitude so that the low frequency component in the output opticalsignal is to be smaller. By so doing, the operation state of the SMZM 4is optimized. In addition, the phase of the low frequency component inthe output optical signal is in phase (zero phase difference) orreversed (it phase difference) with respect to the low frequency signalLf generated by the low frequency modulator 36. The phase indicates thedirection (increase or decrease) of the control of the bias voltageand/or drive amplitude. Note that it may be alright to say that thecontroller 30 controls the bias voltage and/or drive amplitude based onthe phase difference (0 or π) between the low frequency component andthe low frequency signal Lf.

The bias controller 40 controls the bias voltages V1bias and V2bias. Inthis case, the bias controller 40 controls the bias voltages V1bias andV2bias so that the low frequency component in the output optical signalis reduced based on the output of the phase detector 38. The lowfrequency signal Lf is superimposed on the bias voltages V1bias andV2bias as necessary. The bias voltages V1bias and V2bias are controlledbased on the power (and phase) of the low frequency component in theoutput optical signal. In this case, the bias controller 40 controls thebias voltage in the X-axis direction and/or Y-axis direction. That is,the bias controller 40 generates a set of bias voltages V1bias andV2bias so that the bias voltage in the X-axis direction and/or Y-axisdirection are optimized or substantially optimized based on the lowfrequency component detected by the phase detector 38.

A drive amplitude controller 42 generates an amplitude control signal Vcfor controlling the amplitude of the modulation signal generated by thedriver 28. The drive amplitude controller 42 generates the amplitudecontrol signal Vc so that the low frequency component in the outputoptical signal is reduced based on the output of the phase detector 38.The low frequency signal Lf is superimposed on the amplitude controlsignal Vc as necessary. In this case, the voltage superimposed with thelow frequency signal Lf is applied for the driver 28. That is, themodulation amplitudes of the in-phase modulation signal V1 pp and thereversed-phase modulation signal V2 pp are controlled based on the power(and phase) of the low frequency component in the output optical signal.

Thus, the controller 30 automatically controls the bias voltage in theX-axis direction and the Y-axis direction and drive amplitude bycontrolling a pair of bias voltages V1bias and V2bias and the amplitudecontrol signal Vc based on the low frequency component in the outputoptical signal of the SMZM 4. Therefore, the operation state of the SMZM4 is optimized or substantially optimized without individually adjustingin advance the bias voltage and drive amplitude of the SMZM 4.

With the control, the bias voltage and drive amplitude are automaticallyadjusted for a constantly optimum or nearly optimum state even with thefluctuation of the characteristics of the SMZM 4 caused by a temperaturechange, a change by aging, etc. or the variance etc. of an LSI or acircuit device including the driver 28, the controller 30, the opticalmonitor 34, etc.

Next, refer to FIG. 2 for the procedure of the modulation control. FIG.2 illustrates an example of an optical modulation control method. Theprocedure illustrated in FIG. 2 is an example of the method of opticalmodulation control according to the present application.

In the procedure in FIG. 2, a modulation signal and a bias voltage areapplied to the optical waveguides 6 and 8. Thus, the input light of theoptical waveguides 6 and 8 is modulated. (S11)

The optical monitor 34 detects the output optical signal of the SMZM 4.The monitor result is sent to the controller 30. (S12)

The controller 30 detects the low frequency component of the outputoptical signal. In this case, the controller 30 detects at least thepower of the low frequency component. The controller 30 may detect thepower and phase of the low frequency component. (S13)

Based on the detected low frequency component, the controller 30controls the amplitude of the modulation signal and the bias voltage.(S14)

Thus, the optical waveguides 6 and 8 are provided with the modulationsignal and the bias voltage controlled above. The processes in S11-S14are repeatedly performed. Therefore, the SMZM 4 can continuously performthe optical modulation (phase modulation) in the optimum or nearlyoptimum operation state. That is, even when various fluctuation factorssuch as a temperature change, a change by aging, a variance of device,etc. exist, the optical modulator according to the present embodimentprovides a stable modulation operation, and generates a high qualityoptical signal.

In the embodiment above, the low frequency component is detected by thephase detector 38, but the present invention is not limited to thisconfiguration. That is, since the output optical signal of the SMZM 4includes the information about the operation state of the SMZM 4, thepresent invention may extract the information about the operation stateof the SMZM 4 in another method. For example, the present invention maymonitor the low frequency component in the output optical signal of theSMZM 4 or its harmonic components and control the SMZM 4 depending onthe monitor result by synchronous detection of the low frequencycomponent using the low frequency signal Lf.

In the embodiment described above, both of the modulation signal andbias voltage provided for the signal electrodes 16 and 18 arecontrolled, but the present invention may be configured to control oneof the drive amplitude and the bias voltage.

With the automatic control as described above, the bias voltage anddrive amplitude can be optimized or nearly optimized, thereby obtainingthe optical modulation output without the influence of a change by agingetc. As a result, the optical modulation output can be stabilized.

Next, refer to FIG. 3 for the static characteristic and the optimumdrive of the SMZM. FIG. 3 illustrates an example of the staticcharacteristic of the SMZM, and an example of the optimum driving methodof the SMZM. (A) of FIG. 3 illustrates the static characteristic (levelline expression) of the SMZM. (B) and (C) of FIG. 3 are eye-diagrams ofthe waveforms of the input voltage V1 and the input voltage V2,respectively.

FIG. 3 is a 3-dimensional graph of the operation characteristic of theSMZM 4. The horizontal axis indicates the input voltage V1 applied tothe signal electrode 16 of the optical waveguide 6. The vertical axisindicates the input voltage V2 applied to the signal electrode 18 of theoptical waveguide 8. The output optical power of the SMZM 4 is expressedusing the level line in the direction vertical to the surface of thesheet of FIG. 3. The level line expression is normalized. That is, themaximum optical power of the SMZM 4 is expressed by “1.0”, and theminimum optical power (or extinguished state) of the SMZM 4 is expressedby “0.0”.

To maximize the modulation level of the optical signal in the opticalphase modulation, the modulation is performed so that the drive stateobtained by a pair of input voltages V1 and V2 moves between two peakpoints (that is, the points where the optical power is “1.0”) asillustrated in (A) of FIG. 3. To obtain the modulation operation, theamplitude voltage Vpp (that is, amplitude voltage Vpp of the in-phasemodulation signal V1 pp and the reversed-phase modulation signal V2 pp)and the bias voltages V1bias and V2bias of the modulation signal appliedto the optical waveguides 6 and 8 are to be controlled so that theoperation state is optimized. For example, the amplitudes of themodulation signals V1 pp and V2 pp and the bias voltages V1bias andV2bias are controlled so that the data signal “1” is set at one peakpoint (optical power=1.0), and the data signal “0” is set at anotherpeak point (optical power=1.0). The modulation signal V1 pp indicates,for example, “zero” or “π”, and the modulation signal V2 pp indicates,for example, “zero” or “−π”.

In this specification, the direction parallel to the virtual straightline connecting the two peak points of the static characteristicsillustrated in (A) of FIG. 3 is referred to as a “Y axis” or a “Y-axisdirection”. In the static characteristic indicated in (A) of FIG. 3, thedirection parallel to the line indicating the optical power of zero isreferred to an “X axis” or an “X-axis direction”. In the SMZM, the Xaxis and the Y axis are orthogonal or approximately orthogonal to eachother. In (A) of FIG. 3, the broken lines L1 and L2 indicate the levelsat which “fold-back” occurs in the waveform of the output optical signalof the SMZM.

The operation state of the SMZM 4 is controlled so that applied voltagemoves between the two peak points illustrated in (A) of FIG. 3. That is,the operation state obtained by the voltage corresponding to themodulation signal moves between the two peak points illustrated in (A)of FIG. 3. Therefore, in the specification, the direction parallel tothe straight line connecting two peak points is referred to as a“modulation direction”. That is, the modulation direction is the Y-axisdirection, and the X-axis direction is orthogonal to the modulationdirection.

Next, refer to FIG. 4 for explanation of the optical modulation in theY-axis direction. FIG. 4 illustrates an example of a modulatingoperation in the Y-axis direction. (A) of FIG. 4 indicates the opticaloutput characteristic in the Y-axis direction (that is, the modulationdirection). (B) of FIG. 4 illustrates the waveform (eye diagram) of amodulation signal. (C) of FIG. 4 illustrates the waveform (eye diagram)of an output optical signal.

In the optical phase modulation, the drive amplitude in the Y-axisdirection is 2Vπ as illustrated in (A) and (B) of FIG. 4. That is, themodulation signal “0” indicates the voltage corresponding to one of thetwo peak points of the output optical power of the SMZM 4. Themodulation signal “1 (π or −π)” indicates the voltage corresponding tothe other peak point of the output optical power of the SMZM 4. Thepower of the output optical signal of the SMZM 4 is 1.0 as illustratedin (C) of FIG. 4. The power of the output optical signal of the SMZM 4becomes temporarily zero when the value of the modulation signalchanges.

Next, refer to FIGS. 5 and 6 for the optical output characteristic inthe Y-axis direction. As with FIG. 3, FIG. 5 is a 3-dimensional graph ofthe optical output characteristic. FIG. 6 indicates the optical outputcharacteristic corresponding to the voltage change in the Y-axisdirection. FIG. 7 illustrates an example of the optical output powercharacteristic in the X-axis direction.

When the bias voltage of the SMZM 4 changes, the optical outputcharacteristic of the SMZM 4 changes correspondingly. For example, inthe optical output characteristic illustrated in FIG. 5, it is assumedthat four operation states a, b, c, and d of different bias voltages areapplied to the SMZM 4. The state b corresponds to the optimum state. Inthis case, as illustrated in FIG. 6, the optical output characteristicsa, b, c, and d are obtained. Each of the optical characteristics a, b,c, and d in FIG. 6 respectively correspond to the operation states a, b,c, and d in FIG. 5.

When the bias voltage is adjusted in the optimum state b, the outputoptical power is 1.0. In this case, as illustrated by the characteristicb in FIG. 6, high optical output is obtained. When the bias voltage isshifted from the optimum value in the X-axis direction, the outputoptical power of the SMZM 4 is reduced. For example, since the state aillustrated in FIG. 5 is located near the peak point of the opticalpower, the output optical power of the SMZM 4 is not reduced largely(characteristic a in FIG. 6). On the other hand, since the states c andd in FIG. 5 are apart from the peak point of the optical power, theoutput optical power of the SMZM 4 is largely reduced (characteristics cand d in FIG. 6).

Thus, when the bias voltage is shifted in the X-axis direction withrespect to the optimum value, the output optical power of the SMZM 4 isreduced, and the modulation efficiency is degraded. In the SMZM 4, asillustrated in FIG. 7, when the bias voltage is shifted from the optimumvalue in the X-axis direction, the peak value of the optical output isreduced. That is, when the bias voltage is controlled for the optimumvalue, the maximum optical output power is obtained. Therefore, in theSMZM 4, a preferable phase modulation is realized by optimizing theamplitudes of the modulation signals V1 pp and V2 pp and the biasvoltages V1bias and V2bias applied to the optical waveguides 6 and 8.

According to this control, excellent optical modulation output can beobtained although the bias map of the SMZM 4 has X-axis directiondependence (change in optical intensity and level line interval). Thatis, the bias voltage and the drive amplitude can approach the optimumpoint, and a stable optical modulation output is obtained. In addition,it is not necessary to measure or acquire in advance the optimum driveamplitude and optimum bias voltage for each wavelength of carrier light.

The static characteristic of the SMZM 4 is variable for each device. Inaddition, as described above, the static characteristic changes withrespect to the wavelength of carrier light. However, according to theconfiguration and method of the embodiments, the drive amplitude and thebias voltage can be optimized or approximately optimized by the feedbackcontrol based on the output optical signal. Therefore, it is notnecessary to make an adjustment for each optical modulator to obtain theoptimum point of the drive amplitude and the bias voltage. Furthermore,although the characteristics of the SMZM 4 are changed by temperaturechange, a change by aging, etc., the drive amplitude and the biasvoltage can be optimized during the operation of the optical modulator.Therefore, the degradation of an optical signal by the fold-back ofwaveform of an optical waveform, the degradation of extinction ratio, across point fluctuation, the reduction of the aperture of an opticalwaveform, etc. can be avoided.

<Effect of the First Embodiment>

-   (1) Since a bias voltage and a drive amplitude are automatically    adjusted according to the output optical signal of the SMZM 4, the    bias voltage and the drive amplitude can be free of adjustments.    Furthermore, it is not necessary to perform a process or an    operation for acquiring data for the optimum bias and the optimum    amplitude for each wavelength of carrier light in advance.-   (2) Since a bias voltage and a drive amplitude are optimized or    approximately optimized, the influence of a change by aging and a    characteristic change of an optical modulator of the SMZM 4 etc. and    its peripheral circuits (driver 28, controller 30, etc.) can be    avoided, thereby preventing the degradation of an optical waveform.

<Second Embodiment>

Refer to FIG. 8 for the second embodiment. FIG. 8 illustrates an exampleof an optical transmission module according to the second embodiment.The configuration illustrated in FIG. 8 is an example, and the presentinvention is not limited to the configuration. In FIG. 8, the samecomponent as in FIG. 1 is assigned the same reference numeral.

An optical transmission module 200A illustrated in FIG. 8 is an exampleof the optical modulator, the optical transmitter, and the opticalmodulation control method according to the present application. Theoptical transmission module 200A includes the SMZM 4, the controller 30,the driver 28, and the optical monitor 34 as with the optical modulator2 illustrated in FIG. 1.

The optical transmission module 200A is provided with a light source 44at the input side of the SMZM 4, and the output light of the lightsource 44 is guided to the SMZM 4. The light source 44 can be, forexample, a DC light source. The light source 44 generates, for example,CW (continuous wave) light. The SMZM 4 includes the input terminals 20and 22. The input terminal 20 is electrically coupled to the signalelectrode 16 for applying an electric field to the optical waveguide 6,and the second input terminal 22 is electrically coupled to the signalelectrode 18 for applying an electric field to the optical waveguide 8.The input voltages V1 and V2 are provided for the input terminals 20 and22, respectively. The SMZM 4 modulates the phase of the transmissionlight of the optical waveguides 6 and 8 by the refractive indexmodulation by the electro-optical effect. As a result, a modulatedoptical signal is generated. The output optical signal of the SMZM 4 ismonitored by the optical monitor 34. The optical monitor 34 includes,for example, an optical splitter 46 and a photo detector 48. In thiscase, the optical splitter 46 branches a part of the output opticalsignal of the SMZM 4 and guides the optical signal to the photo detector48. The photo detector 48 converts the branched optical signal into anelectric signal by generating a current depending on the branchedoptical signal. The photo detector 48 includes, for example, aphotodiode.

The controller 30 includes a current/voltage (I/V) converter 50. The I/Vconverter 50 converts a current signal into a voltage signal. The I/Vconverter 50 can be configured to detect a low frequency componentincluded in the optical signal. In this case, the I/V converter 50obtains a voltage signal indicating the low frequency component in theoutput optical signal. The I/V converter 50 is realized by, for example,a transimpedance amplifier.

The output signal of the I/V converter 50 is guided to a phasecomparator 52 of the phase detector 38. The phase comparator 52 detectsthe low frequency component in the output optical signal using the lowfrequency signal Lf generated by the low frequency modulator 54. Anintegrator 56 detects the power and the phase of the low frequencycomponent in the output optical signal by integrating (that is,averaging) the output signal of the phase comparator 52. The integrator56 provides the function of smoothing the output signal of the phasecomparator 52 and removing the high frequency component. The integrator56 may be configured by including, for example, a low pass filter. Whenthe power of the low frequency component in the output optical signal isdetected by the phase comparator 52, the integrator 56 can be omitted.

The low frequency modulator 54 is an example of a signal source of thelow frequency signal Lf as a reference signal, and generates the lowfrequency signal Lf having a substantially constant amplitude. Thefrequency of the low frequency signal Lf is sufficiently lower than thebit rate or a symbol rate of the input data signal. In addition, it isassumed that the amplitude of the low frequency signal Lf issufficiently smaller, than the amplitude (that is, the drive amplitude)of the modulation signal output from a driver circuit 76. The lowfrequency signal Lf generated by the low frequency modulator 54 isguided to the phase comparator 52, an adder 74, an adder 68, and apolarity switch 72 as necessary.

The controller 30 further includes an X-axis direction bias controller(first bias controller) 58, a Y-axis direction bias controller (secondbias controller) 60, a drive amplitude controller 62, a V1biascontroller 64, and a V2bias controller 66.

The X-axis direction bias controller 58 controls the bias point of theSMZM 4 in the X-axis direction on the map illustrated in FIG. 3 so thatthe output signal of the integrator 56 (that is, the low frequencycomponent in the output optical signal) is zero or minimized in thesequence of controlling the bias in the X-axis direction. That is, theX-axis direction bias controller 58 adjusts the bias point of the SMZM 4in the X-axis direction by controlling the input voltages V1 and V2. Inthis case, the adjustment direction of the bias point on the X axis maybe determined by the polarity (that is, positive or negative) of theoutput voltage of the integrator 56. Then, the bias voltages V1bias andV2bias respectively generated by the V1bias controller 64 and the V2biascontroller 66 are controlled so that the output of the integrator 56 iszero or minimized. For example, when it is assumed that the amount ofthe change of the voltage of the bias voltage V1bias is ΔV1, and theamount of the change of the voltage of the bias voltage V2bias is ΔV2,the bias point moves in the X-axis direction when ΔV1=ΔV2. In thesequence of controlling the bias in the X-axis direction, ΔV1 and ΔV2are calculated by the X-axis direction bias controller 58 according tothe output signal of the integrator 56.

The Y-axis direction bias controller 60 controls the bias point of theSMZM 4 in the Y-axis direction so that the output signal of theintegrator 56 (that is, the low frequency component in the outputoptical signal) is zero or minimized in the sequence of controlling thebias in the Y-axis direction. That is, the Y-axis direction biascontroller 60 adjusts the bias point of the SMZM 4 in the Y-axisdirection by controlling the input voltages V1 and V2. In this case, theadjustment direction of the bias point on the Y axis may be determinedby the polarity (that is, positive or negative) of the output voltage ofthe integrator 56. Then, the bias voltages V1bias and V2biasrespectively generated by the V1bias controller 64 and the V2biascontroller 66 are controlled so that the output of the integrator 56 iszero or minimized. For example, when it is assumed that the amount ofthe change of the voltage of the bias voltage V1bias is ΔV1, and theamount of the change of the voltage of the bias voltage V2bias is ΔV2,the bias point moves in the X-axis direction when ΔV1=−ΔV2. In thesequence of controlling the bias in the Y-axis direction, ΔV1 and ΔV2are calculated by the Y-axis direction bias controller 60 according tothe output signal of the integrator 56.

The V1bias controller 64 generates the bias voltage V1bias based on theoutput of the X-axis direction bias controller 58 and the Y-axisdirection bias controller 60. The V1bias controller 64 calculates thenext bias voltage V1bias based on the current bias voltage V1bias, theoutput of the X-axis direction bias controller 58, and the output of theY-axis direction bias controller 60. That is, the bias voltage V1bias iscontrolled according to the low frequency component in the outputoptical signal, and the bias voltage V1bias is optimized so that the lowfrequency component is zero or minimized.

The operation of the V2bias controller 66 is similar to the operation ofthe V1bias controller 64. However, the V2bias controller 66 generatesthe bias voltage V2bias based on the output of the X-axis direction biascontroller 58 and the output of the Y-axis direction bias controller 60.

The adder 68 adds the low frequency signal Lf to the output signal ofthe V1bias controller 64. That is, the low frequency signal Lf issuperimposed on the bias voltage V1bias. An adder 70 adds the outputsignal of the polarity switch 72 to the output signal of the V2biascontroller 66. The polarity switch 72 reverses the polarity (or phase)of the low frequency signal Lf when a reverse instruction is receivedfrom the control circuit described later. That is, the low frequencysignal Lf or the reversed low frequency signal Lf is superimposed on thebias voltage V2bias. In the sequence of controlling the bias in theX-axis direction, the polarity switch 72 does not reverse the lowfrequency signal Lf, thus the low frequency signal Lf is superimposed onthe bias voltage V2bias. On the other hand, in the sequence ofcontrolling the bias in the Y-axis direction, the polarity switch 72reverses the low frequency signal Lf, thus the reversed low frequencysignal Lf is superimposed on the bias voltage V2bias.

The drive amplitude controller 62 generates an amplitude control voltageVc for controlling a drive amplitude so that the output signal of theintegrator 56 (that is, the low frequency component of the outputoptical signal) is zero or minimized in the sequence of controlling thedrive amplitude. In this case, the adjustment direction of the driveamplitude (that is, whether the drive amplitude is to be larger orsmaller) may be determined by the polarity (that is, positive ornegative) of the output voltage of the integrator 56.

The adder 74 adds the low frequency signal Lf to the output signal ofthe drive amplitude controller 62 in the sequence of controlling thedrive amplitude. That is, the low frequency signal Lf is superimposed onthe amplitude control voltage Vc. When the control of the driveamplitude is not performed, the low frequency signal Lf is not providedfor the adder 74, and the amplitude control voltage Vc output from theadder 74 is a DC voltage.

The driver circuit 76 is an example of the driver 28 illustrated in FIG.1, and generates a modulation signal (in-phase modulation signal V1 pp,reversed-phase modulation signal V2 pp) corresponding to the input datasignal. The driver circuit 76 controls the amplitude (that is, driveamplitude) of the modulation signal based on the amplitude controlvoltage Vc provided by the controller 30. Therefore, in the sequence ofcontrolling the drive amplitude, the amplitudes of the modulationsignals V1 pp and V2 pp dithers in synchronization with the lowfrequency signal Lf.

The in-phase modulation signal V1 pp and the bias voltage V1bias arecombined by the bias T circuit, and the resultant signal is provided asthe input voltage V1 for the input terminal 20. The reversed-phasemodulation signal V2 pp and the bias voltage V2bias are combined by thebias T circuit, and the resultant signal is provided as the inputvoltage V2 for the second input terminal 22. Each of the bias T circuitsinclude a capacitor 78 electrically coupled to the driver circuit 76,and an inductor 80 electrically coupled to the controller 30. Thecapacitor 78 provides high impedance for the bias voltages V1bias andV2bias. The inductor 80 provides high impedance for the in-phasemodulation signal V1 pp and the reversed-phase modulation signal V2 pp.Therefore, these combined signals are provided efficiently for the inputterminals 20 and 22, respectively. The bias T circuits can be replacedby another circuit having a similar function.

Each of the adders 68, 70, and 74 operates as a superimposer. In thiscase, the superimposer may include the low frequency modulator 54. Thesuperimposer may further include the polarity switch 72. The X-axisdirection bias controller 58, the Y-axis direction bias controller 60,the drive amplitude controller 62, the V1bias controller 64, and theV2bias controller 66 operate as a “controller to control the biasvoltage in the modulation direction, the bias voltage in the orthogonaldirection, and the amplitude of a modulation signal”. In this case, thecontroller may include the phase comparator 52 and the integrator 56.

Next, refer to FIGS. 9 and 10 for the bias control in the Y-axisdirection. FIG. 9 illustrates an example of an operation when a lowfrequency signal is superimposed on the bias voltage in the Y-axisdirection. (A) of FIG. 9 illustrates the static characteristics of theSMZM 4. (B) of FIG. 9 illustrates a waveform of the bias voltage V1bias.(C) of FIG. 9 illustrates a voltage waveform in the Y-axis direction.(D) of FIG. 9 illustrates a waveform of the bias voltage V2bias. (E) ofFIG. 9 illustrates a voltage waveform in the X-axis direction. FIG. 10illustrates the phase of each waveform illustrated in (B)-(E) of FIG. 9.

In the bias control in the Y-axis direction, as illustrated in (B) ofFIG. 9, an in-phase low frequency signal (non-reversed waveform) issuperimposed on the input voltage V1. In addition, as illustrated in (D)of FIG. 9, a reversed-phase low frequency signal (reversed waveform) issuperimposed on the input voltage V2. The bold arrow indicated in (A) ofFIG. 9 refers to a change of the bias voltage in the Y-axis directiondue to the low frequency signal.

Thus, when the in-phase low frequency signal and the reversed-phase lowfrequency signal, having the same amplitude, are superimposed on theinput voltages V1 and V2, respectively, the low frequency signal iscanceled in the X-axis direction. Therefore, as indicated by (E) of FIG.9, the bias voltage in the X-axis direction is not changed. On the otherhand, as indicated by (C) of FIG. 9, the voltage in the Y-axis directionis modulated by the low frequency signal.

In this case, as illustrated in FIG. 10, the in-phase low frequencysignal Lf is superimposed on the input voltage V1 and the reversed-phaselow frequency signal Lf is superimposed on the input voltage V2. As aresult, since the in-phase low frequency signal Lf and thereversed-phase low frequency signal Lf are canceled, the bias voltage inthe X-axis direction becomes a DC voltage at approximately constantlevel as illustrated in FIG. 10. On the other hand, the bias voltage inthe Y-axis direction dithers at the same frequency as the low frequencysignal Lf as illustrated in FIG. 10. The phase of the bias voltage inthe Y-axis direction may be synchronous with the reversed-phase lowfrequency signal Lf.

Refer to FIGS. 11 and 12 for the bias control in the Y-axis direction.FIG. 11 illustrates an example of a bias control operation in the Y-axisdirection. (A) of FIG. 11 illustrates the input/output characteristicsof the SMZM 4. (B) of FIG. 11 illustrates the waveform of the datasignal modulation signal. (C) of FIG. 11 illustrates the waveform of theoutput optical signal. FIG. 12 illustrates an example of the biasvoltage in the Y-axis direction, the data signal modulation signal, andthe output optical signal in the operation illustrated in FIG. 11.

In this example, the data signal modulation signal indicated by (B) ofFIG. 11 is applied to the SMZM 4 having the operation characteristicindicated by (A) of FIG. 11. The data signal modulation signal isprovided as the input voltages V1 and V2 for the SMZM 4. In the sequenceof controlling the bias in the Y-axis direction, the bias voltagesV1bias and V2bias are modulated by the low frequency signal Lf.Therefore, the data signal modulation signal includes the component ofthe low frequency signal Lf as indicated by (B) of FIG. 11. The SMZM 4outputs the optical signal indicated by (C) of FIG. 11.

In the sequence of controlling the bias in the Y-axis direction, asindicated by (A) of FIG. 12, the bias voltage in the Y-axis direction ismodulated by the low frequency signal Lf. That is, the bias voltage inthe Y-axis direction dithers at the frequency as the low frequencysignal Lf. Therefore, the data signal modulation signal includes thecomponent of the low frequency signal Lf as indicated by (B) of. FIG.12.

However, when the bias voltage in the Y-axis direction is optimized, thelow frequency component in the optical output becomes zero as indicatedby (C) of FIG. 12. In this case, the output optical signal includesdouble component of the frequency of the low frequency signal Lf. On theother hand, when the bias voltage in the Y-axis direction is shiftedtoward the positive side from the optimum value, the output opticalsignal includes the reversed-phase low frequency component with respectto the low frequency signal Lf as indicated by (D) of FIG. 12. When thebias voltage in the Y-axis direction is shifted toward the negative sidefrom the optimum value, the output optical signal includes the in-phaselow frequency component with respect to the low frequency signal Lf asindicated by (E) of FIG. 12.

Therefore, the controller 30 can determine the adjustment direction ifthe low frequency component in the output optical signal is detected andthe phase of the detected low frequency component is compared with thatof the low frequency signal Lf. That is, when the low frequencycomponent having the same phase as the low frequency signal Lf isdetected, the controller 30 shifts the bias voltage in the Y-axisdirection in the positive direction, thereby making the bias voltageapproach the optimum point. Similarly, when the low frequency componenthaving the reversed phase with respect to the low frequency signal Lf isdetected, the controller 30 makes the bias voltage approach the optimumpoint by shifting the bias voltage in the Y-axis direction in thenegative direction. According to this feedback control, the bias voltageapproaches the optimum point. Meanwhile, this feedback control issubstantially equivalent to the control of reducing the low frequencycomponent in the output optical signal. Therefore, the controllerminimizes the low frequency component in the output optical signal, andthus optimizes the bias voltage in the Y-axis direction.

Next, refer to FIGS. 13 and 14 for the drive amplitude control in theY-axis direction. FIG. 13 illustrates the operation when the lowfrequency signal is superimposed on the amplitude control voltage Vc.(A) of FIG. 13 illustrates the static characteristic of the SMZM 4. (B)of FIG. 13 illustrates the waveform of the input voltage V1. (C) of FIG.13 illustrates the voltage waveform in the Y-axis direction. (D) of FIG.13 illustrates the waveform of the input voltage V2. (E) of FIG. 13illustrates the voltage waveform in the X-axis direction. FIG. 14illustrates the phase of the waveform indicated by (B)-(E) of FIG. 13.

In the drive amplitude control in the Y-axis direction, for example, thelow frequency signal Lf is superimposed on the amplitude control voltageVc. As a result, as indicated by (B) and (D) of FIG. 13, the lowfrequency signal Lf is superimposed on each of the input voltages V1 andV2. That is, the input voltages V1 and V2 are amplitude-modulated by thelow frequency signal Lf. Note that, in (A) of FIG. 13, the arrowindicated by double lines refers to a voltage change in the Y-axisdirection.

In the control of the drive amplitude in the Y-axis direction, thecontroller 30 superimposes the low frequency signal Lf on the amplitudecontrol voltage Vc. As a result, as indicated by (C) of FIG. 13, thevoltage modulated by the low frequency signal Lf in the Y-axis directionis generated. However, in this embodiment, a pair of the data signalmodulation signals generated by the driver circuit 76 form adifferential signal. Therefore, in the X-axis direction, the lowfrequency signal Lf is canceled. Accordingly, the voltage in the X-axisdirection is approximately DC voltage at a constant level as indicatedby (E) of FIG. 13.

In the sequence of controlling the drive amplitude, the low frequencysignal Lf is superimposed on the amplitude control voltage Vc asindicated by (A) of FIG. 14. The driver circuit 76 is driven by theamplitude control voltage Vc. Thus, the in-phase data signal modulationsignal indicated by (B) of FIG. 14 and the reversed-phase data signalmodulation signal indicated by (C) of FIG. 14 are generated. The datasignal modulation signal is a symmetrical amplitude modulation signalindicating the same change on the high and low potential sides.Therefore, the voltage signal in the Y-axis direction isamplitude-modulated by the low frequency signal Lf as indicated by (D)of FIG. 14.

Refer to FIGS. 15 and 16 for the drive amplitude control in the Y-axisdirection. FIG. 15 illustrates an example of the drive amplitude controlin the Y-axis direction. (A) of FIG. 15 illustrates the input/outputcharacteristics of the SMZM 4. (B) of FIG. 15 illustrates the waveformof the data signal modulation signal. (C) of FIG. 15 illustrates thewaveform of the output optical signal. FIG. 16 illustrates an example ofthe amplitude control voltage Vc, the data signal modulation signal, andoutput optical signal in the operation illustrated in FIG. 15.

In this example, the data signal modulation signal illustrated in (B) ofFIG. 15 is input to the SMZM 4 having the input/output characteristicindicated by (A) of FIG. 15. In the sequence of controlling the driveamplitude, the drive amplitude voltage Vc is modulated by the lowfrequency signal Lf. Therefore, the amplitude of the data signalmodulation signal includes the component of the low frequency signal Lfas indicated by (B) of FIG. 15. Thus, the SMZM 4 outputs the opticalsignal illustrated in (C) of FIG. 15.

As indicated by (A) of FIG. 16, the amplitude control voltage Vc ismodulated by the low frequency signal Lf. The driver circuit 76generates the data signal modulation signal from the input data signalbased on the amplitude control voltage Vc. Therefore, the amplitude ofthe data signal modulation signal is symmetrical amplitude modulatedaccording to the low frequency signal Lf. With the input/outputcharacteristic illustrated in (A) of FIG. 15, when the voltage in theY-axis direction exceeds a voltage for the peak output optical power ofthe SMZM 4, the “fold-back” occurs in the output optical signal waveformof the SMZM 4. (C) of FIG. 15 and (C) of FIG. 16 illustrate the waveformof the optical signal in which the fold-back occurs in the samefrequency as the low frequency signal Lf.

When the drive amplitude is optimized in the sequence of controlling thedrive amplitude in the Y-axis direction, the low frequency component inthe output optical signal is zero as indicated by (C) of FIG. 16. Inthis case, the output optical signal includes double frequency componentof the low frequency signal Lf. On the other hand, when the driveamplitude is smaller than the optimum value, the output optical signalincludes the in-phase low frequency component of the low frequencysignal Lf as indicated by (D) of FIG. 16. When the drive amplitude islarger than the optimum value, the output optical signal includes thereversed-phase low frequency component with respect to the low frequencysignal Lf as indicated by (E) of FIG. 16.

Therefore, the controller 30 can determine the adjustment direction bydetecting the low frequency component in the output optical signal, andby comparing the phases between the detected low frequency component andthe low frequency signal Lf. That is, when the controller 30 detects alow frequency component having the same phase as the low frequencysignal Lf, the controller 30 increases the drive amplitude to make thedrive amplitude approach the optimum value. When the controller 30detects a low frequency component having the reversed phase with respectto the low frequency signal Lf, the controller 30 reduces the driveamplitude to make the drive amplitude approach the optimum value. Inthis operation, the feedback control is substantially equivalent to thecontrol of reducing the low frequency component in the output opticalsignal. Therefore, this feedback control minimizes the low frequencycomponent in the output optical signal, thereby optimizing the driveamplitude.

Next, refer to FIGS. 17 and 18 for the bias control in the X-axisdirection. FIG. 17 illustrates an example of the bias control operationin the X-axis direction. (A) of FIG. 17 illustrates the staticcharacteristic of the SMZM 4. (B) of FIG. 17 illustrates the waveform ofthe bias voltage V1bias. (C) of FIG. 17 illustrates the voltage waveformin the Y-axis direction. (D) of FIG. 17 illustrates the waveform of thebias voltage V2bias. (E) of FIG. 17 illustrates the voltage waveform inthe X-axis direction. FIG. 18 illustrates the bias voltage V1bias, thebias voltage V2bias, the voltage in the X-axis direction, and thevoltage in the Y-axis direction in the operation illustrated in FIG. 17.

In the bias control in the X-axis direction, the low frequency signal Lfis superimposed on the bias voltage in the X-axis direction. The boldarrow on the map indicated by (A) of FIG. 17 represents the voltagedithering for the bias control in the X-axis direction. In the biascontrol, the bias voltage V1bias on which the low frequency signal Lf issuperimposed as illustrated in (B) of FIG. 17 and the bias voltageV2bias on which the low frequency signal Lf is superimposed asillustrated in (D) of FIG. 17 are used. In this case, the voltage in theY-axis direction is not changed by the low frequency signal Lf asillustrated in (C) of FIG. 17. On the other hand, as illustrated in (E)of FIG. 17, the voltage in the X-axis direction is modulated by the lowfrequency signal Lf.

The superimposition of the low frequency signal Lf on the voltage in theX-axis direction is realized by superimposing the low frequency signalLf of the same phase on both of the bias voltages V1bias and V2bias.That is, as illustrated in (A) and (B) of FIG. 18, when the biasvoltages V1bias and V2bias are amplitude-modulated according to the lowfrequency signal Lf of the same amplitude and the same phase, the lowfrequency signal Lf is canceled in the Y-axis direction. As a result, asillustrated in (D) of FIG. 18, the voltage in the Y-axis direction isapproximately DC voltage at a constant level. On the other hand, thevoltage in the X-axis direction is modulated by the low frequency signalLf as illustrated in (C) of FIG. 18.

Further refer to FIGS. 19 and 20 for the control of the X-axis bias.FIG. 19 illustrates an example of an operation of controlling the X axisbias. (A) of FIG. 19 indicates the optical output characteristic withrespect to the voltage in the X-axis direction. (B) of FIG. 19 indicatesthe voltage waveform in the X-axis direction. (C) of FIG. 19 indicatesthe waveform of the output optical signal. FIG. 20 illustrates the biasvoltage in the X-axis direction and the optical output power in theoperation in FIG. 19.

In this example, the voltage signal in the X-axis direction indicted by(B) of FIG. 19 is input to the SMZM 4 having the characteristicindicated by (A) of FIG. 19. In the sequence of controlling the biasvoltage in the X-axis direction, the voltage in the X-axis direction ismodulated by the low frequency signal Lf. The SMZM 4 outputs the opticalsignal indicated by (C) of FIG. 19. FIG. 19 illustrates the state inwhich the bias voltage in the X-axis direction is optimized.

In the sequence of controlling the bias voltage in the X-axis direction,the voltage in the X-axis direction is modulated by the low frequencysignal Lf as indicated by (A) of FIG. 20. When the bias voltage in theX-axis direction is optimum, the low frequency component in the outputoptical signal is zero as indicated by (B) of FIG. 20. In this case, theoutput optical signal includes double frequency component of the lowfrequency signal Lf. When the bias voltage in the X-axis direction isshifted from the optimum to the positive side, the output optical signalincludes the reversed-phase low frequency component with respect to thelow frequency signal Lf. When the bias voltage in the X-axis directionis shifted from the optimum value to the negative side, the outputoptical signal includes the in-phase low frequency component withrespect to the low frequency signal Lf.

Therefore, the controller 30 can determine the direction of adjustmentby detecting the low frequency component in the output optical signaland comparing the phases between the detected low frequency componentand the low frequency signal Lf. That is, when the controller 30 detectsthe in-phase low frequency component with respect to the low frequencysignal Lf, the controller 30 shifts the bias voltage in the X-axisdirection to the positive direction to make the bias voltage approachthe optimum point. Similarly, when the controller 30 detects thereversed-phase low frequency component with respect to the low frequencysignal Lf, the controller 30 shifts the bias voltage in the X-axisdirection to the negative direction to make the bias voltage approachthe optimum point. This feedback control is substantially equivalent tothe control of reducing the low frequency component in the outputoptical signal. Therefore, this feedback control reduces the lowfrequency component in the output optical signal, thereby optimizing thebias voltage in the X-axis direction.

As described above, the controller 30 of the optical transmission module200A includes the low frequency modulator 54 for generating the lowfrequency signal Lf. The controller 30 modulates the bias voltage of theSMZM 4 in the X-axis direction by the low frequency signal Lf in thesequence of controlling the bias voltage in the X-axis direction.Similarly, the controller 30 modulates the bias voltage of the SMZM 4 inthe Y-axis direction by the low frequency signal Lf in the sequence ofcontrolling the bias voltage in the Y-axis direction. In addition, thecontroller 30 may amplitude-modulates the modulation signal for drivingthe SMZM 4 by the low frequency signal Lf.

The optical monitor 34 includes the optical splitter 46 for branching apart of the output optical signal of the SMZM 4, and the photo detector48 for converting the branched optical signal into an optical current.The controller 30 includes the I/V converter 50 to convert the currentsignal generated by the photo detector 48 into a voltage signal. Thus,the optical monitor 34 monitors the output optical signal, and theelectric signal indicating the output optical signal is provided for thecontroller 30, thereby realizing high efficiency drive control.

The controller 30 has the phase detector 38 including the phasecomparator 52 and the integrator 56. The phase detector 38 detects thepower and the phase of the low frequency component included in theoutput optical signal.

The controller 30 includes the X-axis direction bias controller 58, theY-axis direction bias controller 60, and the drive amplitude controller62, and performs the control of the bias voltage in the X-axisdirection, the control of the bias voltage in the Y-axis direction, andthe control of the amplitude of a modulation signal based on the lowfrequency component in the output optical signal. According to thiscontrol operation, the amplitude of the modulation signal and the biasvoltage are optimized or approximately optimized.

<Third Embodiment>

Refer to FIG. 21 for the third embodiment. FIG. 21 illustrates anexample of a configuration of the optical transmission module accordingto the third embodiment. In FIG. 21, the same components illustrated inFIGS. 1 and 8 are assigned the same reference numerals.

An optical transmission module 200B illustrated in FIG. 21 is an exampleof the optical modulator, the optical transmitter, and the opticalmodulation control method according to the present application. Theoptical transmission module 200B includes the SMZM 4, the controller 30,the driver 28 (driver circuit 76), and the optical monitor 34. The lightsource 44 is provided at the input side of the SMZM 4.

The controller 30 of the optical transmission module 200B performs thesuperimposition of the low frequency signal Lf, the control of the biasvoltage, and the control of the drive amplitude by the time divisionscheme. In the controller 30, the low frequency signal Lf generated bythe low frequency modulator 54 is guided to the phase comparator 52, andlow frequency switches 92, 94, and 96. The low frequency switches 92,94, and 96 are controlled into the ON state or the OFF state at theinstruction from the time division controller 98. The low frequencyswitches 92, 94, and 96 pass the low frequency signal Lf in the ONstate. The low frequency switches 92, 94, and 96 block the low frequencysignal Lf in the OFF state. Therefore, when the low frequency switches92, 94, and 96 are controlled into the ON state, the low frequencysignal Lf is guided to the adders 74, 68, and 70, respectively. However,the polarity switch 72 is provided between the low frequency switch 96and the adder 70. Thus, when a reverse instruction is issued from thetime division controller 98, the reversed low frequency signal Lf isprovided to the adder 70.

The time division controller 98 controls the operation of each elementin the controller 30 by time division scheme. That is, the time divisioncontroller 98 controls the operation of the X-axis direction biascontroller 58, the Y-axis direction bias controller 60, the driveamplitude controller 62, and the polarity switch 72. The time divisioncontroller 98 controls the state of the low frequency switches 92, 94,and 96. Furthermore, the time division controller 98 provides theoperation mode of modulating the bias voltage by the low frequencysignal Lf and the operation mode of modulating the amplitude of themodulation signal by the low frequency signal Lf by controlling thepolarity switch 72. The time division controller 98 includes, forexample, a clock circuit to switch the operation mode of the controller30 at specified time intervals.

Refer to FIG. 22 for the controlling operation of the opticaltransmission module 200B. FIG. 22 is a flowchart of the procedure of thecontrolling operation. In the control procedure of the opticaltransmission module 200B, the bias control in the Y-axis direction(S31), the Y axis amplitude control (S32), and the bias control in theX-axis direction (S33) are repeatedly performed.

In the bias control in the Y-axis direction (S31), the time divisioncontroller 98 controls the low frequency switches 92, 94, and 96 intothe OFF state, the ON state, and the ON state, respectively. The timedivision controller 98 controls the polarity switch 72 to perform thereverse operation (ON state: reverse operation). Furthermore, the timedivision controller 98 controls the Y-axis direction bias controller 60into the operating state (ON), and controls the X-axis direction biascontroller 58 into the non-operating state (OFF: maintaining the currentsetting). The time division controller 98 controls the drive amplitudecontroller 62 into non-operating state (OFF). Thus, during S31 isperformed, the amplitude of the modulation signal is fixed.

In S31, the control operation explained with reference to FIGS. 9-12 isrealized. That is, the adder 68 superimposes the low frequency signal Lfon the bias voltage V1bias, and the adder 70 superimposes the reversedlow frequency signal Lf on the bias voltage V2bias. In this case, thebias voltage in the Y-axis direction is modulated by the low frequencysignal Lf as illustrated in (D) of FIG. 10. Therefore, the outputoptical signal of the SMZM 4 includes the frequency component (that is,the low frequency component) of the low frequency signal Lf. The Y-axisdirection bias controller 60 controls the V1bias controller 64 and theV2bias controller 66 to reduce the low frequency component in the outputoptical signal. As a result, the bias voltage in the Y-axis direction isoptimized or approximately optimized.

In the Y axis amplitude control (S32), the time division controller 98controls the low frequency switches 92, 94, and 96 into the ON state,the OFF state, and the OFF state, respectively. The time divisioncontroller 98 controls the Y-axis direction bias controller 60 and theX-axis direction bias controller 58 into the non-operating state (OFF:maintaining the current setting). Thus, during S32 is performed, thebias voltage (both in X-axis direction and Y-axis direction) applied tothe SMZM 4 is fixed. Note that when the Y axis amplitude is controlled,the low frequency switch 96 is controlled into the OFF state, and thelow frequency signal Lf is not guided to the polarity switch 72.Therefore, the polarity switch 72 can be in the ON state or the OFFstate.

In S32, the operation explained with reference to FIGS. 13-16 isrealized. That is, the adder 74 superimposes the low frequency signal Lfon the amplitude control voltage Vc. In this case, the amplitude of themodulation signal generated by the driver circuit 76 is modulated by thelow frequency signal Lf as indicated by (B) and (C) of FIG. 14.Therefore, the output optical signal of the SMZM 4 includes the lowfrequency component. The drive amplitude controller 62 controls theamplitude control voltage Vc to reduce the low frequency component. As aresult, the drive amplitude is optimized or approximately optimized.

In the bias control in the X-axis direction (S33), the time divisioncontroller 98 controls the low frequency switches 92, 94, and 96 intothe OFF state, the ON state, and the ON state, respectively. Inaddition, the time division controller 98 controls the polarity switch72 not to perform the reverse operation (OFF: non-reverse operation).Furthermore, the time division controller 98 controls the Y-axisdirection bias controller 60 into the non-operating state (OFF), andcontrols the X-axis direction bias controller 58 into the operatingstate (ON). The time division controller 98 controls the drive amplitudecontroller 62 into non-operating state (OFF). Thus, during S33 isperformed, the amplitude of the modulation signal is fixed.

In S33, the operation explained with reference to FIGS. 17-20 isrealized. That is, the adder 68 superimposes the low frequency signal Lfon the bias voltage V1bias, and the adder 70 superimposes the lowfrequency signal Lf on the bias voltage V2bias. In this case, the biasvoltage in the X-axis direction is modulated by the low frequency signalLf as illustrated in FIG. 18. Therefore, the output optical signal ofthe SMZM 4 includes the low frequency component. The X-axis directionbias controller 58 controls the V1bias controller 64 and the V2biascontroller 66 to reduce the low frequency component. As a result, thebias voltage in the X-axis direction is optimized or approximatelyoptimized.

By performing the process in S32, the bias voltage in the Y-axisdirection optimized in S31 may be shifted from the optimum value. Inaddition, by performing the process in S33, the bias voltage in theY-axis direction optimized in S31, and/or drive amplitude optimized inS32 may be shifted from the optimum value. Therefore, the controller 30may repeatedly perform the processes in S31-S33. In this case, thecontroller 30 may repeatedly perform the processes in S31-S33 for aspecified number of times. In addition, the controller 30 may repeatedlyperform the processes in S31-S33 until each of the bias voltage in theY-axis direction, the drive amplitude, and the bias voltage in theX-axis direction sufficiently converges.

Thus, in the third embodiment, the Y-axis direction bias controller 60,the X-axis direction bias controller 58, and the drive amplitudecontroller 62 selectively operate under the time division control by thetime division controller 98. In the third embodiment, the bias in theX-axis direction, the bias in the Y-axis direction, and the amplitude ofthe modulation signal are controlled using one low frequency signal Lf.In this case, the low frequency signal Lf superimposed on one of thebias voltages V1 and V2 is used as is in the operation mode forcontrolling the X axis bias, and is used after reversed by the polarityswitch 72 in the operation mode for controlling the Y axis bias.

Furthermore, in the third embodiment, the bias voltage in the X-axisdirection, the bias voltage in the Y-axis direction, and the driveamplitude of the modulation signal are respectively controlled indifferent time section. Therefore, the accuracy of each controllingoperation is high.

<Fourth Embodiment>

Refer to FIG. 23 for the fourth embodiment. FIG. 23 illustrates anexample of a configuration of the optical modulator according to thefourth embodiment. In FIG. 23, a element also appearing in FIG. 21 isassigned the same reference numeral.

An optical transmission module 200C illustrated in FIG. 23 is an exampleof the optical modulator, the optical transmitter, and the opticalmodulation control method according to the present application. Theoptical transmission module 200C includes the SMZM 4, the controller 30,the driver 28 (driver circuit 76), and the optical monitor 34illustrated in FIG. 1. The light source 44 is provided at the input sideof the SMZM 4.

In the controller 30 of the optical transmission module 200C, lowfrequency signals LF1, LF2, and Lf3 of different frequencies f1, f2, andf3 are used. That is, the above-mentioned bias voltage control in theY-axis direction, the bias voltage control in the X-axis direction, andthe drive amplitude control are performed using these three lowfrequency signals LF1, LF2, and Lf3. In the embodiment, the lowfrequency signal Lf1 is used for control of the bias voltage in theX-axis direction, the low frequency signal Lf2 is used for control ofthe bias voltage in the Y-axis direction, and the low frequency signalLf3 is used for control of the amplitude of the modulation signal.

The controller 30 includes a first low frequency modulator 541, a secondlow frequency modulator 542, and a third low frequency modulator 543.The first low frequency modulator 541 generates the low frequency signalLf1 of the frequency f1, the second low frequency modulator 542generates the low frequency signal Lf2 of the frequency f2, and thethird low frequency modulator 543 generates the low frequency signal Lf3of the frequency f3. The frequencies f1, f2, and f3 are not specificallyrestricted, but may be, for example, f1=1 kHz, f2=1.3 kHz, and f3=1.6kHz.

The low frequency signal Lf1 is superimposed on the output signal of theV1bias controller 64 by an adder 681. The low frequency signal Lf1 isalso superimposed on the output signal of the V2bias controller 66 by anadder 701. The low frequency signal Lf2 is reversed by a polarityinverter 722, and then the reversed low frequency signal Lf2 issuperimposed on the output signal of the V1bias controller 64 by anadder 682. Furthermore, the low frequency signal Lf2 is superimposed onthe output signal of the V2bias controller 66 by an adder 702. The lowfrequency signal Lf3 is superimposed on the output signal of the driveamplitude controller 62 by the adder 74. Therefore, the data, signalmodulation signal output from the driver circuit 76 isamplitude-modulated by the low frequency signal Lf3. Then, the SMZM 4 isprovided with the bias voltage and the modulation signal on which thelow frequency signals LF1, LF2, and Lf3 are superimposed.

To detect each low frequency component corresponding to the lowfrequency signals LF1, LF2, and Lf3, the controller 30 includes phasedetectors 381, 382, and 383. Each of the phase detector 381, 382, and383 includes a phase comparator and an integrator 561.

The phase detector 381 detects the frequency component (Lf1 component)which is the same as the low frequency signal Lf1 from the output signalof the I/V converter 50 using the low frequency signal Lf1. Similarly,the phase detector 382 detects the frequency component (Lf2 component)which is the same as the low frequency signal Lf2 from the output signalof the I/V converter 50 using the low frequency signal Lf2. The phasedetector 383 detects the frequency component (Lf3 component) which isthe same as the low frequency signal Lf3 from the output signal of theI/V converter 50 using the low frequency signal Lf3.

The Lf1 component detected by the phase detector 381 is provided for theX-axis direction bias controller 58. The X-axis direction biascontroller 58 controls the V1bias controller 64 and the V2biascontroller 66 to reduce the Lf1 component. That is, the bias voltagesV1bias and V2bias is controlled so that the Lf1 component is reduced.Similarly, the Lf2 component detected by the phase detector 382 isprovided for the Y-axis direction bias controller 60. The Y-axisdirection bias controller 60 controls the V1bias controller 64 and theV2bias controller 66 to reduce the LF2 component. That is, the biasvoltages V1bias and V2bias are controlled so that the Lf2 component canbe reduced. Furthermore, the Lf3 component detected by the phasedetector 383 is provided fro the drive amplitude controller 62. Thedrive amplitude controller 62 controls the amplitude control voltage Vcto reduce the Lf3 component.

According to the fourth embodiment, the bias voltage control in theY-axis direction, the bias voltage control in the X-axis direction, andthe control of the amplitude of the modulation signal are performedusing the low frequency signals Lf1 through Lf3 of differentfrequencies. Therefore, in the fourth embodiment, the bias voltagecontrol in the Y-axis direction, the bias voltage control in the X-axisdirection, and the control of the amplitude of the modulation signal canbe performed in parallel. In addition, since the low frequency signalshaving different frequencies are used, the detection accuracy of theoptical output characteristic can be enhanced, thereby realizing highaccuracy control.

<Fifth Embodiment>

Refer to FIG. 24 for the fifth embodiment. FIG. 24 illustrates anexample of a configuration of a bias controller according to the fifthembodiment. In FIG. 24, the same element appearing in FIGS. 8, 21, and23 is assigned the same reference numerals.

The X-axis direction bias controller 58, the Y-axis direction biascontroller 60, the V1bias controller 64, and the V2bias controller 66 inFIG. 24 are used for the above-mentioned controller 30. The X-axisdirection bias controller 58 is realized in this example by a reverseamplification circuit including an operation amplifier 580 and resistors582 and 584. In this case, the X-axis direction bias controller 58outputs a signal of the level depending on the amount of voltage changeof the output signal of the integrator 56 of the phase detector 38. TheY-axis direction bias controller 60 includes in this example a reverseamplification circuit 606 and amplifier 608. The reverse amplificationcircuit 606 includes an operation amplifier 600 and resistors 602 and604. The amplifier 608 obtains non-reversed output and reversed output.In this case, the Y-axis direction bias controller 60 outputs a signalat the level depending on the amount of voltage change of the output ofthe integrator 56 of the phase detector 38, and its reversed signal.

The V1bias controller 64 is realized by, for example, a resistor networkcircuit 644 including resistors 640 and 642. The V1bias controller 64 isprovided with a signal output from the X-axis direction bias controller58 and a reversed signal output from the Y-axis direction biascontroller 60. The resistor network circuit 644 generates the biasvoltage V1bias by combining two input signals. Similarly, the V2biascontroller 66 is realized by, for example, a resistor network circuit664 including resistors 660 and 662. In this case, the V2bias controller66 is provided with a signal output from the X-axis direction biascontroller 58, and a non-reversed signal output from the Y-axisdirection bias controller 60. The resistor network circuit 664 generatesthe bias voltage V2bias by combining two input signals.

<Variations>

(1) In the embodiments above, a semiconductor Mach-Zehnder modulator(SMZM) is exemplified as an optical modulator, but the present inventionis not limited to this configuration. That is, the optical modulator canbe realized by other modulators.

(2) The present invention can be applied to any phase modulating scheme(binary phase modulation, multilevel phase modulation, polarizationmultiplexed multilevel phase modulation, etc.).

(3) In the embodiments above, the bias voltages in the modulationdirection and the direction orthogonal to the modulation direction arecontrolled for phase modulation. However, the optical modulator of theinvention does not have to control the bias in both directions. That isto say, the optical modulator of the invention controls the bias in atleast one of the two directions.

(4) In the embodiments above, the bias control of the modulationdirection, the bias control in the orthogonal direction, and the controlof the drive amplitude are performed. However, the optical modulator ofthe invention does not have to perform all of the three operation modes.That is to say, the optical modulator of the invention may perform onlyone or two of the three operation modes.

(5) The optical modulator/optical transmission module may be configuredcontrol the bias only in the X-axis direction by performing an in-phaselow frequency modulating on a differential bias voltage.

(6) The optical modulator/optical transmission module may be configuredto control the bias only in the Y-axis direction by performing areversed-phase low frequency modulating on a differential bias voltage.

(7) The optical modulator/optical transmission module may be configuredto control only one of the X-axis direction bias and Y-axis directionbias, and control the drive amplitude.

(8) The driver circuit 76 and/or the controller 30 may be configured bya digital circuit. In this case, the digital circuit may include acomputer, a PLD (programmable logic device), an FPGA (field programmablegate array) etc.

(9) In the third embodiment, time division scheme is performed using onelow frequency signal Lf. In the fourth embodiment, control is performedin the frequency division scheme using the low frequency signals Lf1through Lf3 of different frequencies. However, the present invention isnot limited to these control schemes. That is, the time division schemesimilar to the third embodiment may be performed while using a pluralityof low frequency signals of different frequencies. In addition, when aplurality of low frequency signals are used, the levels of the lowfrequency signals may be different.

<Comparison Example>

Refer to FIG. 25 for a comparison example. FIG. 25 illustrates a SMZMand its peripheral circuits. The optical modulator of the comparisonexample includes the above-mentioned SMZM 4. In FIG. 25, the sameelements in FIGS. 1 and 8 are assigned the same reference numerals.

A driver circuit 280 is electrically coupled to the input terminals 20and 22 of the SMZM 4. In addition, and the bias control circuits 130 and132 are electrically coupled to the input terminals 20 and 22,respectively. An amplitude control circuit 134 is electrically coupledto the driver circuit 280. The waveforms 136, 138, and 140 are waveformsof the input data signal, the input voltage V1, and the input voltageV2, respectively.

The output optical signal of the SMZM 4 is generated by combining thetransmission light of the optical waveguides 6 and 8. Therefore, thepower of the output optical signal depends on the phases of a pair oflight beams passing the optical waveguides 6 and 8. That is, when thephases of a pair of light beams passing the optical waveguides 6 and 8are the same, the power of the output optical signal is the maximum.When the phases of the pair of the light beams are opposite each other,the power of the output optical signal is the minimum.

In the SMZM 4, the refractive indexes of the optical waveguides 6 and 8change with the respective input voltages V1 and V2, respectively. Thatis, the phases of the light which passes the optical waveguides 6 and 8change with the input voltages V1 and V2, respectively. In the phasemodulation, the SMZM 4 is driven by the push-pull scheme (differentialscheme) to suppress the optical frequency chirp.

In the phase modulation using the SMZM 4, an optimum drive amplitude andoptimum bias voltage are determined in advance for each SMZM device andfor each wavelength of carrier light, and the determined operationcondition is set in the optical modulator. However, there are thefollowing problems with the configuration in which the setting is made.

As described above, with the configuration in which the operationcondition is set for each SMZM device and for each wavelength of thecarrier light, it takes long time to determine the optimum operationcondition.

In addition, the optimum operation condition of the SMZM may fluctuateby various factors. For example, the static characteristic of the SMZMcan be changed by a temperature change or aging of a device etc. Inaddition, when the characteristics of the bias control circuits 130 and132, the driver circuit 280, and the amplitude control circuit 134 arechanged, the drive amplitude and the bias voltage may fluctuate. If thedrive amplitude or the bias voltage is shifted from the optimum point bythese factors, there occur the fold-back of an optical waveform, thedegradation of extinction ratio, the fluctuation of cross point, and thereduction of the aperture of an optical waveform, thereby degrading thequality of the optical waveform. These problems have been solved by theconfiguration and the control method according to the embodiments of thepresent application.

In the LN (LiNbO₃) optical modulator, the bias control and the driveamplitude control are performed in the Y-axis direction, but no controlis performed in the X-axis direction. However, the static characteristicof the SMZM has the X axis dependency. Therefore, if the controlperformed for the LN optical modulator is introduced to the SMZM, it isdifficult or impossible to obtain the optimum operation condition of theSMZM.

<QPSK Modulation>

FIG. 26 illustrates a configuration of an optical transmission moduleprovided with a QPSK modulator. An optical transmission module 300includes a QPSK modulator 301. The QPSK modulator 301 includes an SMZM 4a, an SMZM 4 b, and a phase shifter 302. The configurations of the SMZM4 a and the SMZM 4 b are substantially the same as that of the SMZM 4illustrated in FIG. 1. However, input signals V1 and V2 are applied tothe SMZM 4 a, and input signals V3 and V4 are applied to the SMZM 4 b.The QPSK modulator 301 is formed on, for example, one semiconductorchip. However, the terminators 24 and 26 illustrated in FIG. 27 may beprovided outside the semiconductor chip. In this case, the semiconductorchip and the terminators 24 and 26 are coupled by a bonding wire.

The phase shifter 302 provides a phase difference π/2 between theoptical path which passes through the SMZM 4 a and the optical pathwhich passes through the SMZM 4 b. The QPSK modulator 301 combines thephase modulated optical signals generated by the SMZMs 4 a and 4 b togenerate a QPSK modulated optical signal.

The optical splitter 46 branches the QPSK modulated optical signal. Thephoto detector 48 converts the QPSK modulated optical signal guided fromthe optical splitter 46 into a current signal. The I/V converter 50converts the current signal generated by the photo detector 48 into avoltage signal. That is, the I/V converter 50 generates an electricsignal indicating the QPSK modulated optical signal.

The phase controller 303 uses the output of the I/V converter 50 (thatis, uses the QPSK modulated optical signal generated by the QPSKmodulator 301), and optimizes the amount of phase shift of the phaseshifter 302 to π/2. The method of adjusting the phase shifter 302 is notspecifically limited, but can be performed by the method described inJapanese Laid-open Patent Publication No. 2007-82094.

The low frequency modulator 54 generates a low frequency signal Lf. Inaddition, the phase comparator 52 and the integrator 56 uses the lowfrequency signal Lf to detect the low frequency component in the QPSKmodulated optical signal.

An amplitude/bias controller 304 a controls the bias voltage of the SMZM4 a and the amplitude of the modulation signal of the modulation signalto drive the SMZM 4 a based on the low frequency component in the QPSKmodulated optical signal. Likewise, an amplitude/bias controller 304 bcontrols the bias voltage of the SMZM 4 b and the amplitude of themodulation signal to drive the SMZM 4 b based on the low frequencycomponent in the QPSK modulated optical signal. A time divisioncontroller 305 controls the operations of the amplitude/bias controllers304 a and 304 b.

FIG. 27 illustrates a configuration of the QPSK modulator 301illustrated in FIG. 26. The QPSK modulator 301 includes the SMZMs 4 aand 4 b, the phase shifter 302, an optical splitter 311, and an opticalcombiner 312.

The optical splitter 311 branches the input CW light and guides the CWlight to the SMZMs 4 a and 4 b. Each of the SMZMs 4 a and 4 b aresubstantially the same as the SMZM 4 illustrated in FIG. 1 etc. However,the signal electrodes 16 and 18 of the SMZM 4 a are provided with theinput voltages V1 and V2, respectively. The input voltage V1 isgenerated by adding the bias voltage V1bias output from theamplitude/bias controller 304 a to the modulation signal output from thedriver circuit 76 a. The input voltage V2 is generated by adding thebias voltage V2bias output from the amplitude/bias controller 304 a tothe reversed modulation signal output from the driver circuit 76 a.Similarly, the input voltages V3 and V4 are respectively applied to thesignal electrodes 16 and 18 of the SMZM 4 b. The input voltage V3 isgenerated by adding the bias voltage V3bias output from theamplitude/bias controller 304 b to the modulation signal output from thedriver circuit 76 b. The input voltage V4 is generated by adding thebias voltage V4bias output from the amplitude/bias controller 304 b tothe reversed modulation signal output from the driver circuit 76 b. Theoptical combiner 312 combines the optical signals generated by the SMZMs4 a and 4 b. With the configuration, the QPSK modulated optical signalis generated.

FIGS. 28A and 28B illustrate a control system of the opticaltransmission module 300 illustrated in FIG. 26. In FIGS. 28A and 28B,only the amplitude/bias controllers 304 a and 304 b are illustrated as acontrol system of the optical transmission module 300.

The amplitude/bias controller 304 a includes the X-axis direction biascontroller 58 a, the Y-axis direction bias controller 60 a, the driveamplitude controller 62 a, the V1bias controller 64 a, the V2biascontroller 66 a, the adders 68 a, 70 a, and 74 a, the polarity switch 72a, and the low frequency switches 92 a, 94 a, and 96 a. Similarly, theamplitude/bias controller 304 b includes the X-axis direction biascontroller 58 b, the Y-axis direction bias controller 60 b, the driveamplitude controller 62 b, the V3bias controller 64 b, the V4biascontroller 66 b, the adders 68 b, 70 b, and 74 b, the polarity switch 72b, and the low frequency switches 92 b, 94 b, and 96 b. The operationsof the amplitude/bias controllers 304 a and 304 b are substantially thesame as those of the X-axis direction bias controller 58, the Y-axisdirection bias controller 60, the drive amplitude controller 62, theV1bias controller 64, the V2bias controller 66, the adders 68, 70, and74, the polarity switch 72, the low frequency switches 92, 94, and 96.However, the amplitude/bias controller 304 a generates the bias voltagesV1bias and V2bias and the amplitude control voltage Vca. Theamplitude/bias controller 304 b generates the bias voltages V3bias andV4bias and the amplitude control voltage Vcb.

FIG. 29 is a flowchart of the control method of the optical transmissionmodule 300 illustrated in FIG. 26. The process of the flowchart isrealized by the time division controller 305 issuing an instruction tothe amplitude/bias controllers 304 a and 304 b.

The processes in S41-S43, and the processes in S44-S46 are substantiallythe same as those in S31-S33 in FIG. 22. However, the processes inS41-S43 are performed on the amplitude/bias controller 304 a to controlthe operating state of the SMZM 4 a. The processes in S44-S46 areperformed on the amplitude/bias controller 304 b to control theoperating state of the SMZM 4 b.

In the embodiment illustrated in FIG. 29, the bias voltage of the QPSKmodulator 301 and the amplitude of the modulation signal are controlledin time division scheme, but the present invention is not limited tothis scheme. That is, the bias voltage of the QPSK modulator 301 and theamplitude of the modulation signal may be controlled in parallel by, forexample, a frequency division scheme.

The embodiments of the optical modulator, the optical transmitter, andthe optical modulation control method are described above, but thepresent invention is not limited to the descriptions above, and thoseskilled in the art can modify and vary in various ways based on the gistof the present invention described in the scope of the claims for thepatent and disclosed in the embodiments. The modifications andvariations are included in the scope of the present invention.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiment (s) of the presentinventions has (have) been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. An optical modulator, comprising: a modulatorincluding an optical waveguide provided in a semiconductor substratehaving an electro-optical effect and an electrode to apply an electricfield depending on a bias voltage and a modulation signal to the opticalwaveguide; a driver circuit to generate a modulation signal inaccordance with an input signal; a superimposer to superimpose areference signal on the bias voltage, the reference signal having lowerfrequency than the modulation signal; and a controller to control a biasvoltage in a modulation direction of the modulator and an orthogonaldirection orthogonal to the modulation direction based on the frequencycomponent of the reference signal extracted from a modulated opticalsignal generated by the modulator, wherein the optical waveguideincludes first and second optical waveguides forming a Mach-Zehnderinterferometer, the electrode includes first and second electrodes toapply an electric field to the first and second optical waveguides,respectively, the modulation signal and a first bias voltage areprovided for the first electrode, a reversed signal of the modulationsignal and a second bias voltage are provided for the second electrode,the controller includes a first bias controller to control a biasvoltage in the orthogonal direction, when the superimposer respectivelysuperimposes the reference signals of same phase on the first biasvoltage and the second bias voltage, the first bias controller controlsthe bias voltage in the orthogonal direction based on the frequencycomponent of the reference signal extracted from the modulated opticalsignal.
 2. The optical modulator according to claim 1, wherein thesuperimposer superimposes the reference signal on an amplitude controlsignal applied to the driver circuit, the controller controls anamplitude of the modulation signal using the amplitude control signalbased on the frequency component of the reference signal extracted fromthe modulated optical signal.
 3. The optical modulator according toclaim 1, wherein the first bias controller shifts the first bias voltageand the second bias voltage by an equal amount in a same direction sothat the frequency component of the reference signal extracted from themodulated optical signal is reduced.
 4. The optical modulator accordingto claim 1, wherein the controller includes a second bias controller tocontrol a bias voltage in the modulation direction, when thesuperimposer respectively superimposes the reference signals of reversedphase from each other on the first bias voltage and the second biasvoltage, the second bias controller controls the bias voltage in themodulation direction based on the frequency component of the referencesignal extracted from the modulated optical signal.
 5. The opticalmodulator according to claim 4, wherein the second bias controllershifts the first bias voltage and the second bias voltage by an equalamount in an opposite direction so that the frequency component of thereference signal extracted from the modulated optical signal is reduced.6. The optical modulator according to claim 1, wherein the controllerincludes an amplitude controller to control an amplitude of themodulation signal using the amplitude control signal, when thesuperimposer superimposes the reference signal on the amplitude controlsignal, the amplitude controller controls an amplitude of the modulationsignal using the amplitude control signal so that the frequencycomponent of the reference signal extracted from the modulated opticalsignal is reduced.
 7. The optical modulator according to claim 1,wherein the controller controls a bias voltage in the modulationdirection, a bias voltage in the orthogonal direction, and an amplitudeof the modulation signal in a time division scheme.
 8. The opticalmodulator according to claim 1, wherein the controller controls a biasvoltage in the modulation direction, a bias voltage in the orthogonaldirection, and an amplitude of the modulation signal in a frequencydivision scheme.
 9. An optical transmission device, comprising: amodulator including an optical waveguide provided in a semiconductorsubstrate having an electro-optical effect and an electrode to apply anelectric field depending on a bias voltage and a modulation signal tothe optical waveguide; a light source to generate carrier light to beinput to the modulator; a driver circuit to generate a modulation signalin accordance with an input signal; a superimposer to superimpose areference signal on the bias voltage and an amplitude control signalprovided for the driver circuit, the reference signal having lowerfrequency than the modulation signal; and a controller to control a biasvoltage in a modulation direction of the modulator, a bias voltage in adirection orthogonal to the modulation direction of the modulator, andan amplitude of the modulation signal based on a frequency component ofthe reference signal extracted from a modulated optical signal generatedby the modulator, wherein the optical waveguide includes first andsecond optical waveguides forming a Mach-Zehnder interferometer, theelectrode includes first and second electrodes to apply an electricfield to the first and second optical waveguides, respectively, themodulation signal and a first bias voltage are provided for the firstelectrode, a reversed signal of the modulation signal and a second biasvoltage are provided for the second electrode, the controller includes afirst bias controller to control a bias voltage in the orthogonaldirection, when the superimposer respectively superimposes the referencesignals of same phase on the first bias voltage and the second biasvoltage, the first bias controller controls the bias voltage in theorthogonal direction based on the frequency component of the referencesignal extracted from the modulated optical signal.
 10. An opticalmodulation control method for controlling an operating state of amodulator including an optical waveguide provided in a semiconductorsubstrate having an electro-optical effect and an electrode to apply anelectric field depending on a bias voltage and a modulation signal tothe optical waveguide, the method comprising: superimposing a referencesignal on the bias voltage, the reference signal having lower frequencythan the modulation signal; and controlling a bias voltage in adirection orthogonal to a modulation direction of the modulator based ona frequency component of the reference signal extracted from a modulatedoptical signal generated by the modulator, wherein the optical waveguideincludes first and second optical waveguides forming a Mach-Zehnderinterferometer, the electrode includes first and second electrodes toapply an electric field to the first and second optical waveguides,respectively, the modulation signal and a first bias voltage areprovided for the first electrode, a reversed signal of the modulationsignal and a second bias voltage are provided for the second electrode,when the reference signals of same phase is superimposed respectively onthe first bias voltage and the second bias voltage, the bias voltage inthe orthogonal direction is controlled based on the frequency componentof the reference signal extracted from the modulated optical signal.